Observation of Internal Waves in the Andaman Sea by ERS SAR
The Andaman Sea is a basin of the Indian Ocean extending between the Peninsula Malayan at the west and the Andaman and Nicobar Islands at the east. At its southeastern side the western entrance of the Malacca Strait is located (Fig. 1).
Since long seafarers passing through the Strait of Malacca on their journeys between India and the Far East have noticed that in the Andaman Sea, between the Nicobar Islands and the north east coast of Sumatra, often bands of strongly increased surface roughness, also called ripplings or bands of choppy water, occur. A description of such bands of choppy water observed from ships in the western approaches of the Malacca Strait can, e.g., be found in the book of Mauray which was published in 1861 and which is quoted in Osborne and Burch : "The ripplings are seen in calm weather approaching from a distance, and in the night their noise is heard a considerable time before they come near. They beat against the sides of a ship with great violence, and pass on, the spray sometimes coming on deck; and by carrying out oceanographic measurements from a ship, a small boat could not always resist the turbulence of these remarkable ripplings".
Perry and Schimke  were the first to show by oceanographic measurements carried out from a ship that these bands of choppy water in the Andaman Sea are associated with large-amplitude oceanic internal waves. Later Osborne and Burch  analyzed oceanographic data collected by the Exxon Production Research Company in the southern Andaman Sea with the aim to assess the impact of underwater current fluctuations associated with oceanic internal waves on drilling operations carried out from a drill ship. They concluded that the visually observed roughness bands are caused by internal solitons which can be described by the Korteweg - de Vries equation .
The oceanographic measurements of Osborne and Burch  showed that the time interval between the first solitons in the packets was typically 40 minutes and then decreased towards the end. In one event, the amplitude (crest-to-trough distance) of the foremost soliton was estimated to be 60 m, i.e., warm water from above was pushed down by the internal soliton by 60 m. The roughness bands associated with one of the internal soliton packets extended from horizon to horizon and were 600 to 1200 m wide. The first band of choppy water consisted of breaking waves about 1.8 m high. The background wave field preceding this band had only a waveheight of 0.6 m. Behind this band of strongly increased surface roughness, the waveheight gradually decreased and a band of reduced surface roughness followed, which had a waveheight of less than 0.1 m and looked "as smooth as a millpond".
Occasionally, long narrow stripes of rough water have also been identified in the Andaman Sea on satellites images acquired in the visible band, e.g., from the Russian-US space station Apollo-Soyus , the Landsat satellites , and the SPOT satellites. However, no systematic investigations of internal solitons could be carried out with these optical satellite images because they can be acquired only sporadically when there are no clouds present and when the sun elevation angle is favourable.
Figure 1. A 900 km long and 100 km wide ERS-2 SAR strip inserted into a map of the Andaman Sea which was acquired on Feb 11, 1997. Visible are sea surface manifestations of several internal solitary wave packets originating from three locations of shallow sea areas which are marked by A, B and C.
Spaceborne radar images showing sea surface manifestations of internal solitons in the Andaman Sea have become available only recently. During the Shuttle Imaging Radar A (SIR-A) mission in 1981, only one SAR image was acquired that shows approximately 50 km east of the North Andaman Island sea surface manifestations of an internal solitary wave packet propagating southwestwards , .
A large number of spaceborne SAR images of the Andaman Sea have recently become available after the ERS receiving station in Singapore became operational in Sept. 1995. These ERS SAR images allow, for the firsttime, to study systematically the spatial distribution of internal solitons in the Andaman Sea and thus obtain information on their generation and propagation characteristics. For our investigation we had available a total of 385 ERS-1/2 SAR scenes of the Andaman Sea each covering an area of 100 km x 100 km.
Since internal solitons are generated at different locations in the Andaman Sea, the solitary wave packets often cross each other. The associated sea surface manifestations show specific interference patterns involving phase shifts. Thus the ERS SAR images can also be used to validate models on the interaction of solitons.
2. GENERATION AREAS
Fig. 1 shows a SAR strip inserted into a map of the Andaman Sea which covers an ocean area of approximately 900 km x 100 km. This SAR strip was acquired by the ERS-2 satellite during orbit 9477 on February 11, 1997, between 03:58 UTC and 03:60 UTC. It is a composite of nine ERS-2 SAR frames (3357, 3375, 3393, 3411, 3429, 3447, 3465, 3483, and 3501). It shows an exceptional wealth of strong sea surface manifestations of internal solitary wave packets. From the curved shape of these patterns one can estimate the position of their focal points. If we assume that the internal solitary wave packets have their origin at these focal points, then one can identify at least three generation areas. They are marked in Fig. 1 by A, B, and C and are located approximately at
(1) the shallow reefs off the northwest coast of Sumatra, around 6° 10N 95° 0E (Indonesian name: Alur Pelayaran Bengala), where, near the 1000 m depth line, a coral reef rises up to a depth of 30 m below the sea surface (position A),
(2) the seamounts at 8° 50N 94° 56E; 9° 04N 94° 34E; and 8° 42N 94° 30E, which have depths of 481 m, 671 m, and 680 m, respectively, and which are located in an ambient sea area which has a depth of more than 2500 m (position B), and
(3) a submarine bank located at 12° 34N 94° 40E which rises from a 1800 m to 2500 m deep oceanfloor to a depth of 88 m below the sea surface (position C).
Note that in the SAR strip shown in Fig. 1 the individual wave patterns are distorted due to interactions between different internal solitary wave packets, which makes it somewhat difficult to locate their focal points exactly, but on several other SAR scenes of the same ocean area the wave patterns are much more regular (although in general, also much fainter) which allow a more accurate location of the generation areas.
Fig. 2 shows an ERS SAR strip inserted into a sea map of the Andaman Sea which covers an ocean area of 500 km x 100 km and which is located further east than the SAR strip shown in Fig. 1. It was acquired by ERS-2 during orbit 5426 on May 4, 1996, between 03:53 UTC and 03:54 UTC and is a composite of five ERS-2 SAR frames (3429, 3447, 3465, 3483, and 3501). Visible is a long-crested ring-shaped roughness pattern which has a length of approximately 300 km. It seems that the focal point is located at the Dreadnought Bank, but the location A shown in the map of Fig. 1 is also consistent with the curvature of this pattern. Also visible on this ERS SAR strip is a second wave pattern in the upper section of the image which we interpret as the sea surface manifestation of an internal solitary wave packet which was generated at the same location one tidal cycle earlier.
Figure 2. A 500 km long and 100 km wide ERS-2 SAR strip inserted into a map of the Andaman Sea, which was acquired on May 4, 1996. Visible are sea surface manifestations of two internal solitary wave packets generated very likely at the same location, but separated in time by one tidal cycle.
In this second solitary wave packet the separation between the solitons is much smaller than in the first wave packet, which results from the interaction of the solitons with the shallow bottom topography in this area. When deviding the distance between the leading solitons of these two packets by the semi-diurnal tidal period (12 hours and 25 minutes) we obtain for the propagation velocity the value 1.43 m/s. However, this value is probably a too low estimate of the propagation velocity of internal solitons in the Andaman Sea because the secondary internal wave packet has propagated through shallow waters where the propagation velocity is smaller than in deep water.
Figure 3. Blow-up of the southern section of the ERS-2 SAR strip shown in Fig. 1. The focal point of the circular wave pattern is located at the shallowest point of the Dreadnought Bank whose position is shown in Fig. 2.
Fig. 3 shows a blow-up of the southern section of the ERS-2 SAR strip shown in Fig. 1 (part of frame 3465). Visible is a ring-shaped pattern that has its focal point at the crosspoint of the two bright lines inserted in this SAR image. This focal point is located at 6°40N 95°47W, which is the position of the shallowest point of the Dreadnough Bank which has a depth of 241 m. This image suggests that a secondary internal solitary wave packet is generated by the interaction of strong internal soliton(s) generated further west with the Dreadnough bank.
3. INTERACTION WITH THE SHELF
Fig. 4 shows another ERS-2 SAR strip covering an area of 200 km x 100 km, which is located farther west than the ERS-2 SAR strip shown in Fig. 2 and which is centered over the 200 m depth line of the western shelf of the Malaysian Peninsula. It was acquired by ERS-2 during orbit 5154 on April 15, 1996, at 03:51 UTC and is a composite of the two SAR frames 3465 and 3483. Visible are sea surface manifestations of two internal solitary wave packets, where one is located east of the 200 m depth line. We interpret them as being generated in the same area, but separated in time by one tidal cycle. The internal solitary wave packet located east of the 200 m depth line has interacted with the shallow bottom topography of the shelf and thus is strongly disturbed, which results in an irregular roughness pattern. However, like the roughness pattern of all other internal solitary wave packets visible on the ERS SAR images presented in this paper, also the roughness pattern of the solitary wave packet east of the 200 m depth line starts with a line of increased surface roughness in front, i.e., with a bright line on the radar image. Thus the front line of the leading soliton is associated with a convergent surface current which implies that the internal solitons east of the 200 m depth line are also solitons of depression.
Fig. 5 shows a scan through the internal wave pattern east of the 200 m line along the line A-B marked in Fig. 4. Plotted is the variation of the normalized radar cross section (NRCS) in decibels (dB). Clearly visible in Fig. 5 is that the NRCS modulation pattern starts with an increase of the NRCS at the front (at the right-hand section in the plot). This scan shows further that the modulation pattern is very irregular which is indicative of perturbations of the internal solitary wave packet due to the interaction with the shelf. Note also that the spatial separation between the solitons within this packet is much smaller than in the other wave packet. We interpret this as being caused by the shoaling of the waves in the shallow bottom sea area.. Another noteworthy feature visible on the ERS SAR image shown in Fig. 4 is the surface roughness pattern associated with the interaction of two internal solitary wave packets. Phase shifts in the roughness pattern can be clearly delineated in regions where the solitary wave packets intersect. Such phase shifts are predicted by soliton theory , .
Figure 4. SAR image of a 200 km x 100 km large section of the Andaman Sea acquired by the ERS-2 satellite on April 15, 1996. It shows sea surface manifestations of two internal solitary wave packets. The one located east of the 200 m depth line has undergone strong interactions of the bottom topography of the shelf area resulting in an irregular roughness pattern.
Figure 5. Variation of the normalized radar cross section along the scan line A-B inserted in the image of Fig. 4.
4. PROPAGATION DIRECTION
As stated before, the propagation direction of internal solitons can be inferred from (1) the curvature and (2) the structure of radar signatures of internal solitons. If an internal soliton is propagating in stratified waters where the upper (lighter) layer is thinner than the lower (heavier) one, then soliton theory predicts that the soliton has to be a soliton of depression. In this case the front of the internal soliton is associated with a region of convergent surface flow and the rear with a region of divergent surface flow. This implies that if short wind generated surface waves are present on the sea surface, then their amplitudes are increased in the convergent and decreased in the divergent regions. Therefore the sea surface manifestation of an internal soliton of depression is imaged by a SAR as a narrow band consisting of a bright front line (high normalized radar cross section) followed by a dark line (low normalized radar cross section) . On the other hand, an internal wave of elevation is imaged by a SAR as a narrow band consisting of a dark line front line followed by a bright line. These internal solitons occur stratified waters, where the upper layer is thicker and the lower one.
On all ERS SAR images of the Andaman Sea analyzed so far, we have found no radar signatures which indicate the presence of internal solitons of elevation.
Our analysis of all available ERS-1/2 SAR data of the Andaman Sea has shown that almost all internal solitons travel in an eastward direction. In particular, we have not found any sea surface manifestations of westward propagating internal solitons in the Andaman Sea east of 96° E.
However, near the submarine bank marked "C" in Fig. 1 some sea surface manifestations of internal solitons propagating in a westward direction have also been delineated (see also the L band SAR image from the SIR-A mission acquired on November 11, 1981, which is discussed in , ).
ERS SAR images of the Andaman Sea are an excellent means to study the spatial distribution and the form of the sea surface manifestation of internal solitons in the Andaman Sea. From them information on their generation and propagation characteristics can be derived. The analysis of 385 ERS-1/2 SAR images of the Andaman Sea has revealed that the internal solitons propagating westwards have their origin at shallow banks in the western part section of the Andaman Sea where they are probably generated by the interaction of the baroclinic tide with shallow bottom topographic features. When solitary wave packets encounter the shallow waters of the western shelf of the Malayan Peninsula, they become heavily disturbed and the seperations between the solitons in the wave packet decreases, but they remain solitons of depression.
This investigation was carried out while W. Alpers was a visiting scientist at the Centre for Remote Imaging, Sensing, and Processing (CRISP) of the National University of Singapore. We thank ESA for providing the ERS SAR data which were processed at CRISP. The work was partially funded by the German Space Agency (DARA) under contract 50 EE 9413.
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Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry
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